This invention relates generally to implantable cardiac stimulating devices. More specifically, the present invention is directed to a dual-chamber cardiac stimulation device using a lead designed for locating the His Bundle, for fixation to the site of the His Bundle, and for sensing a His Bundle signal. This invention further relates to a method for delivering ventricular stimulation at an optimal atrioventricular delay in patients with bundle branch conduction abnormalities.
In a normal human heart, the sinus node, generally located near the junction of the superior vena cava and the right atrium, constitutes the primary natural pacemaker initiating rhythmic electrical excitation of the heart chambers. The cardiac impulse arising from the sinus node is transmitted to the two atrial chambers, causing a depolarization known as a P-wave and the resulting atrial chamber contractions. The excitation pulse is further transmitted to and through the ventricles via the atrioventricular (AV) node and a ventricular conduction system comprised of the bundle of His, the left and right bundle branches, and the Purkinje fibers causing a depolarization known as an R-wave and the resulting ventricular chamber contractions.
Disruption of this natural pacemaking and conduction system as a result of aging or disease can be successfully treated by artificial cardiac pacing using implantable cardiac stimulation devices, including pacemakers and implantable defibrillators, which deliver rhythmic electrical pulses or other anti-arrhythmia therapies to the heart, via electrodes implanted in contact with the heart tissue, at a desired energy and rate. One or more heart chambers may be electrically stimulated depending on the location and severity of the conduction disorder.
A single-chamber pacemaker delivers pacing pulses to one chamber of the heart, either one atrium or one ventricle. Dual chamber pacemakers are now commonly available and can provide stimulation in both an atrial chamber and a ventricular chamber, typically the right atrium and the right ventricle. Both unipolar or bipolar dual chamber pacemakers exist in which a unipolar or bipolar lead extends from an atrial channel of the dual chamber device to the desired atrium (e.g. the right atrium), and a separate unipolar or bipolar lead extends from a ventricular channel to the corresponding ventricle (e.g. the right ventricle). In dual chamber, demand-type pacemakers, commonly referred to as DDD pacemakers, each atrial and ventricular channel includes a sense amplifier to detect cardiac activity in the respective chamber and an output circuit for delivering stimulation pulses to the respective chamber.
If an intrinsic atrial depolarization signal (e.g. a P-wave) is not detected by the atrial channel, a stimulating pulse will be delivered to depolarize the atrium to cause atrial contraction. Following either a detected P-wave or an atrial pacing pulse, the ventricular channel attempts to detect a depolarization signal in the ventricle, known as an R-wave. If no R-wave is detected within a defined atrial-ventricular interval (AV interval, also referred to as AV delay), a stimulation pulse is delivered to the ventricle to cause ventricular contraction. In this way, rhythmic dual chamber pacing is achieved by coordinating the delivery of ventricular output in response to a sensed or paced atrial event.
It is known that the AV delay setting during dual chamber pacing can have profound effects on hemodynamic function, particularly in patients suffering from congestive heart failure. Extreme differences in cardiac output can result from different AV delay settings. The optimal setting varies between individuals and can range from 50 ms to 200 ms. Determining the optimal setting is often difficult since a number of physiological factors influence the hemodynamic function, and hemodynamic measurements required to determine the optimal setting can be costly, time-consuming, and are often invasive with additional inherent risks.
One approach to optimizing the AV delay involves incorporating in the pacemaker system implantable physiological sensors that are capable of detecting a signal that is indicative of the hemodynamic function of the heart. The AV delay can then be adjusted so that the physiological signal measured indicates maximized hemodynamic function. This approach however requires one or more additional sensors, added hardware circuitry, and additional processing time in order to determine the optimal AV delay. Furthermore, the physiological response to a change in AV delay may not be instantaneous but more likely occurs over an extended period of time. Thus, determining the optimal setting may require testing several settings after extended periods of time during which the patient is not receiving optimal stimulation therapy.
In the majority of individuals, the most effective heartbeat is triggered by the patient""s own spontaneous pacemaker. The electronic stimulation device is intended to fill in when the patient""s spontaneous pacemaker fails or when the heart""s conduction system fails. In a large number of heart failure patients, natural conduction through the atrioventricular node and the bundle of His are intact. Disruption of ventricular rhythm in these patients is the result of conduction disorders residing in the left and/or right bundle branches.
Dilatation of the heart due to congestive heart failure (CHF) has been associated with delayed conduction through the ventricles. This delayed conduction exacerbates the hemodynamic inefficiency of the failing heart because of the resulting poor synchronization of the heart chambers.
Direct stimulation of the His Bundle has been found to provide hemodynamic improvement in patients suffering from dilated cardiomyopathy but having normal ventricular activation. Reference is made to Deshmukh P. et al., xe2x80x9cPermanent, Direct His-Bundle Pacingxe2x80x94A New Approach to Cardiac Pacing in Patients With Normal His-Pukinjie Activation,xe2x80x9d Circulation, 2000;101(8)869-77, Feb. 29, 2000. This result supports the hypothesis that the natural conduction system, when intact, can provide hemodynamically optimal depolarization timing of the heart chambers.
What is needed, therefore, is a cardiac stimulation device capable of delivering ventricular stimulation according to the optimal timing dictated by the heart""s own conduction system. It would be desirable, in an implantable dual chamber or multi-chamber cardiac stimulation device, to detect the conduction signal that is naturally conducted through the atrioventricular node and into the His Bundle and trigger ventricular stimulation delivery based on this detected conduction signal.
The present invention addresses this need by providing a cardiac stimulation device and leads, with an associated method for detecting a conduction signal from the His Bundle, and upon this detection, delivering ventricular stimulation to the right and/or left ventricles. According to one embodiment, the system and method of the present invention advantageously deliver ventricular stimulation at an interval following atrial depolarization that is optimally triggered by the functioning portion of the natural conduction system of the heart, and bypasses the dysfunctional portion of the ventricular conduction system to re-establish rhythmic ventricular contractions with the benefit of efficient hemodynamic output.
A preferred embodiment of the present invention provides an implantable cardiac stimulation device equipped with cardiac data acquisition capabilities. The stimulation device includes a control system for controlling the operation of the device; a set of leads with appropriately positioned electrodes for receiving cardiac signals and for delivering atrial and ventricular stimulation pulses; a set of sensing circuits comprised of sense amplifiers for sensing and amplifying the cardiac signals including a modern low noise amplifier used to sense (and optionally to stimulate the His Bundle) and amplify the conduction signal arising from the His Bundle; a sampler, such as an A/D converter for sampling cardiac signals; an averager capable of determining a moving average of a sampled signal; pulse generators for generating atrial and ventricular stimulation pulses; and an impedance measuring circuit for performing a variety of impedance measurements including a tissue impedance measurement used in locating the His Bundle.
In addition, the stimulation device includes a memory for storing operational parameters for the control system, such as cardiac signal sampling parameters and cardiac signal samples. The device also includes a telemetry circuit for communicating with an external programmer. In the preferred embodiment, the stimulation device further includes a physiological sensor of metabolic demand, such as an activity sensor or a minute volume sensor, that provides feedback to the control system which in turn controls the stimulation rate such that the measured metabolic need is met.
The present invention also provides a His Bundle lead. The His Bundle lead includes a tip electrode at the distal end of the lead for sensing His Bundle conduction signals. The tip electrode is provided with an active fixation device for securing the tip electrode to the His Bundle tissue. The tip electrode also includes a non-traumatic conductive surface used to map the location of the His Bundle prior to fixing the tip electrode in the endocardial tissue. It would also elute an acute anti-arrhythmia drug such as lidocaine and/or an anti-inflammatory agent, to reduce the early PVCs that can be caused by the trauma of the lead implantation.
In an exemplary preferred embodiment, the non-traumatic conductive surface is provided as a mapping collar that functions as a source electrode in performing a tissue impedance measurement for locating the His Bundle during the implantation procedure of the stimulation device and His Bundle lead. The His Bundle lead may further include a ring electrode located between approximately 2 mm and 30 mm, but preferably 10 mm, from the tip electrode to be used in bipolar sensing.
During the implantation procedure, the His Bundle lead is advanced into the right atrial chamber and, when the His electrode is positioned proximate the His Bundle tissue, as indicated by an impedance measurement approximately equal to an expected His Bundle tissue impedance, the active fixation electrode is secured in the His Bundle tissue.
When operating according to a preferred embodiment, the stimulation device control system detects an atrial event, either a sensed atrial P-wave or an atrial stimulation pulse, and initiates a His Bundle signal sensing window beginning almost immediately after the atrial event and extending a predefined amount of time, typically 200 ms.
During the sensing window, the His Bundle signal received through the His Bundle lead is sampled and a moving average is calculated to determine a signal peak. Detection of a His Bundle signal peak triggers ventricular stimulation. Ventricular stimulation may be delivered to the right ventricle, the left ventricle, or both ventricles.
The present invention is particularly advantageous for patients with congestive heart failure (CHF) and having normal conduction paths through the AV node and His Bundle but delayed conduction through the bundle branches or the Purkinje system. Such patients tend to benefit from stimulating the left ventricle (via a lead positioned through the coronary sinus with its electrode(s) in a coronary vein overlying the left ventricle) and the right ventricle almost simultaneously. By sensing the His Bundle signal, the stimulation device can deliver biventricular stimulation with optimal AV delays. The delay from the detected His (or His Bundle) signal to the right ventricular stimulation and the delay from the detected His signal to the left ventricular stimulation may be the same or may be programmable values selected by the clinician in order to achieve a desired synchronization between the left and right ventricular contractions.
In an alternative embodiment, if no His signal is detected, ventricular stimulation may be delivered directly to the His Bundle using the His Bundle lead. This alternative embodiment is beneficial to patients having intact conduction below the level of the His Bundle and having intermittent or partial atrioventricular block. When the atrial depolarization is not conducted through the atrioventricular node, the His Bundle lead may be used to deliver stimulation to depolarize the His Bundle. A depolarization of the His Bundle will then be conducted through the ventricles via the left and right bundle branches and the Purkinje fibers to cause ventricular contraction.
The system and method of the present invention thus provide ventricular stimulation optimally timed following an atrial event according to the heart""s natural conduction timing. This optimized ventricular stimulation is accomplished without requiring lengthy testing procedures to determine an optimal AV delay and does not require complex physiological sensors, hardware, or processing algorithms. Patients suffering from congestive heart failure with intact atrioventricular nodal conduction will benefit from improved synchrony of ventricular contractions according to the heart""s natural conduction timing. Patients suffering from intermittent atrioventricular nodal block may benefit from His Bundle stimulation with the stimulation device, by providing a bypass of the block and delivering the ventricular stimulation via the remaining intact conduction pathways.